1. Field of the Invention
The present invention relates to ruthenium-on-charcoal supported catalysts modified with iron, their preparation and their use for the preparation of olefinically unsaturated alcohols by selective hydrogenation of the corresponding carbonyl compounds in the liquid phase in the presence of a noble metal catalyst and of a tertiary amine.
2. Description of the Prior Art
German Laid-Open Application DOS No. 2,934,251 discloses a process for the preparation of olefinically unsaturated alcohols, in particular geraniol and nerol (E- and Z-3,7-dimethyl-octa-2,6-dien-1-ol), by selective hydrogenation of the corresponding .alpha.,62 -unsaturated carbonyl compounds in the liquid phase in the presence of a noble metal catalyst, eg. a ruthenium, rhodium, osmium, iridium or platinum catalyst, and of a tertiary amine. As can be seen from the introduction to the description in the above Laid-Open Application, industrial preparation of the said unsaturated alcohols is fairly difficult, since large amounts of the very expensive noble metal catalysts are necessary, the catalysts must be pretreated in an expensive manner, the activity of the catalyst frequently decreases relatively rapidly, and/or the selectivity of the catalysts leaves much to be desired. The presence of a tertiary amine in the reaction mixture, as proposed in the above Laid-Open Application, substantially increases the selectivity of the reaction and also improves the activity of the catalyst over long operating periods with relatively little expense or technical effort. However, even this process has the following disadvantages, especially if the inexpensive, commercially available ruthenium catalysts are used:
1. The selectivity of the ruthenium catalysts is still not completely satisfactory, especially for the synthesis of geraniol.
2. The hydrogenation does not stop selectively at the alcohol stage after 1 mole of hydrogen has been taken up per mole of aldehyde, and this increases the formation of the perhydrogenation products citronellol and tetrahydrogeraniol, which are difficult to remove by distillation, and thus obstructs preparation of the unsaturated alcohols, which are sought-after scents, in a pure form.
3. The space/time yields in the high pressure range are still unsatisfactory.
4. The hydrogenation times in the pressure range &lt;50 bar are too long when small amounts of catalyst are employed.